The present application claims priority to Japanese Application No. P11-261969 filed Sep. 16, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to methods for fabricating memory devices having a multi-dot floating gate, and in more detail to methods for fabricating memory devices ensuring a desirable crystallization of a semiconductor film without ruining the surface flatness of the polycrystallized silicon layer and a tunnel oxide film, allowing desirable semiconductor dots to be produced, and allowing production of the memory devices having multi-dot floating gate with ease and at low costs even when a substrate is made of glass or plastic.
2. Description of Related Art
In recent years, degree of integration of semiconductor memory devices such as DRAM (dynamic random access memory) and SRAM (static random access memory) keeps on increasing. Such semiconductor memories are the same in-principle in that memory cells thereof are individually composed of transistors, capacitors and the like, the memory cells are connected with each other by wiring through which write operation or read out operation to or from the memory cells are enabled. The semiconductor memories based on such constitution have, however, been suffering from a limited degree of integration.
On the other hand, non-volatile memory devices enabling information read/write operation by light irradiation or application of external electric field principally need no wiring between memory cells thereof, so that they can exempt from limitation on the degree of integration due to wiring and can have a higher degree of integration. As such kind of memory, there is proposed a floating gate MOS (metal oxide semiconductor) memory having a floating gate of the multi-dot type or single dot-type. This floating gate MOS memory is in a full expectation for the future for its long holding time of information.
The conventional floating gate MOS memory is, however, disadvantageous in that it requires a larger cost and longer process time since the silicon dots (silicon thin wires) are formed by photolithography and then shrunk by thermal oxidation (back thermal oxidation) process at high temperatures. The problem also resides in that such heat treatment at high temperatures makes it difficult to fabricate the memory device when the substrate thereof is made of glass or plastic.
The present inventors have proposed in Japanese Patent Application Publication No. 11-274420 a method for fabricating a memory device having a multi-dot floating gate in which an SiO2 film is formed on a substrate, a silicon film is formed on the SiO2 film, and further on the silicon film an Si-excessive non-stoichiometric SiOx (x less than 2) is formed, the silicon film is then laser annealed so as to have a polysilicon structure, and at the same time the SiOx film is decomposed to produce stoichiometric SiO2 and Si, thereby to form silicon dots.
Such method is advantageous in that it can fabricate the memory device having the multi-dot floating gate with ease and at low costs even when the substrate thereof is made of glass or plastic.
In the method disclosed in Japanese Patent Application Publication No. 11-274420, in which the silicon film is converted into the polysilicon film by a single process of the laser annealing, and at the same time the SiOx film is decomposed to produce stoichiometric SiO2 and Si thereby to obtain the silicon dots, a problem resides in that the surface flatness of the polycrystallized silicon layer undesirably ruined by the laser annealing cannot be recovered, and in that a tunnel oxide film may be damaged due to stress caused by shrinkage during the polycrystallization of silicon and the laser annealing with a high-output laser. It was also observed that the silicon film is not sufficiently polycrystallized while the silicon dots are successfully formed, or on the contrary the silicon film is sufficiently polycrystallized while the silicon dots are destroyed.
It is therefore an object of the present invention to provide methods for fabricating memory devices having a multi-dot floating gate ensuring a desirable crystallization of a semiconductor film without ruining the surface flatness of the polycrystallized silicon layer and a tunnel oxide film, allowing desirable semiconductor dots to be produced, and allowing production of the memory devices having a multi-dot floating gate with ease and at low costs even when a substrate is made of glass or plastic.
The present inventors found out after extensive investigations that the pulse energy density required for polycrystallizing the silicon layer is larger than that required for producing the silicon dots, and that the forgoing object can be achieved by a method for fabricating a memory device comprising the steps of: forming on a substrate a semiconductor film and treating said semiconductor film by a first laser annealing so as to have a polycrystalline structure; forming on the semiconductor film a semiconductor dot forming film having a non-stoichiometric composition with an excessive content of a semiconductor element; and dispersing semiconductor dots within the semiconductor dot forming film by a second laser annealing thereby to produce semiconductor dots; in which, a pulse energy density of the laser used for the first laser annealing is larger than a pulse energy density of the laser used for the second laser annealing.
According to the present invention, a laser with a high pulse energy density is used in the first laser annealing in which a large laser pulse energy density is required for polycrystallizing the silicon film, and a laser with an pulse energy density lower than that used in the first laser annealing is used in the second laser annealing in which not so large pulse energy density is required for forming the semiconductor dots. Since the laser anneal conditions are optimally selected individually in the first and second laser annealings, the semiconductor film can be polycrystallized in a desired manner, and semiconductor dots can desirably be dispersed within the semiconductor dot forming film to be provided as semiconductor dots. In this process, the tunnel oxide film is successfully prevented from being damaged by stress caused by the polycrystallization of semiconductor or by the laser annealing with a large pulse energy density. When the flatness of the surface of the polycrystallized semiconductor film is not sufficient, a desirable flatness can be obtained by, for example, the CMP (chemical mechanical polishing) process.
The term xe2x80x9cnon-stoichiometric compositionxe2x80x9d in the context of the present invention refers to a composition expressed by a component ratio deviated from a stoichiometric one.
The term xe2x80x9csemiconductor dotsxe2x80x9d in the context of the present invention refers to semiconductor dots having diameters within a range from 1 nm to 10 nm.
Examples of the-semiconductor dot forming film include silicon dot forming film and germanium dot forming film; examples of such silicon dot forming film include Si-excessive silicon oxide (SiOx) film, and silicon nitride (SiNx) film, and examples of such germanium dot forming film include germanium-excessive germanium oxide (GeOx) film and germanium nitride (GeNx) film.
In the present invention, the first laser annealing is performed using a laser beam with a pulse energy density of 200 to 800 mJ/cm2, more preferably 250 to 550 mJ/cm2, and still more preferably 280 to 450 mJ/cm2.
In the present invention, a laser beam used for the second laser annealing may be selectable depending on the material, thickness and so forth of the semiconductor dot forming film. For a most general semiconductor dot forming film made of SiOx (x=1.2 to 1.9) and has a thickness of 5 to 50 nm, the laser beam preferably has a pulse energy density of 50 to 500 mJ/cm2, more preferably 80 to 400 mJ/cm2, and still more preferably 100 to 300 mJ/cm2.
In a preferable embodiment of the present invention, the semiconductor dot forming film comprises a semiconductor-excessive oxide film or a nitride film.
In a more preferable embodiment of the present invention, the semiconductor dot forming film has a thickness of 5 to 50 nm, and is made of SiOx (x=1.2 to 1.9).
In a more preferable embodiment of the present invention, the semiconductor is selected from the group consisting of Si and Ge as Group IV elements, SiFe2 alloy and SiGe alloy as Group IV compound semiconductors, Group II-VI compound semiconductors and Group III-V compound semiconductors.
In a more preferable embodiment of the present invention, the first and second laser annealings are performed by irradiating excimer laser beam.
In a more preferable embodiment of the present invention, the excimer laser is selected from the group consisting of XeCl excimer laser (wavelength=308 nm), KrF excimer laser (wavelength=248 nm), ArF excimer laser (wavelength=193 nm) and ultraviolet pulse YAG solid-state laser.
In a more preferable embodiment of the present invention, the method further includes the step of forming an insulating layer between the substrate and the semiconductor film is further provided.
The foregoing object can also be achieved by a method for fabricating a memory device comprising: the steps of forming on a substrate a semiconductor film affording a channel region and treating said semiconductor film by a first laser annealing so as to have a polycrystalline structure; forming on the semiconductor film a first insulating film and a semiconductor dot forming film having a non-stoichiometric composition with an excessive content of a semiconductor element stacked in this order; dispersing semiconductor dots within the semiconductor dot forming film by a second laser annealing thereby to produce semiconductor dots; forming on the semiconductor dot forming film having the semiconductor dots produced therein a second insulating film and a control gate stacked in this order; selectively removing the second insulating film, the semiconductor dot forming film having the semiconductor dots produced therein, and the first insulating film, using the control gate as a mask, thereby to form a floating gate; and introducing an impurity into the polycrystallized semiconductor film, in the area along both sides of the floating gate thereby to form a source region and a drain region; in which, a pulse energy density of the laser used for the first laser annealing is larger than a pulse energy density of the laser used for the second laser annealing.